Method for metallizing plastic film

ABSTRACT

A plastic film is double metallized in a vacuum metallizer and a freshly metallized film surface is exposed to air before it is wound into a roll.

United States Patent [191 Craig et a1.

METHOD FOR METALLIZING PLASTIC FILM Inventors: Herbert Craig, Yankeetown, F1a.;

Assignee: Sprague Electric Company, North Adams, Mass.

Filed: Feb. 20, 1973 Appl. No.: 333,925

US. Cl 427/81; 427/209 Int. Cl. C23c 13/02 Field of Search 117/62, 107, 107.1, 71 M, 117/1072 R, 227,103 R, 131

Andrew Dequasie, Barre; Raynor Linzey, Stamford, both of VL; Arthur Webb, Cheshire, Mass.

References Cited UNITED STATES PATENTS Stoll 117/107.1

1 July 15, 1975 McLean et a1 117/107.1 Baer et a1. 117/107.1

Marvin 117/107.1 Peck ll7/107.l Barth 117/107.1 Koller 117/107.1 Hnilicka 1l7/107.1

Primary Examiner-William R. Trenor Attorney, Agent, or FirmConnolly and Hutz ABSTRACT A plastic film is double metallized in a vacuum metal- 1izer and a freshly metallized film surface is exposed to air before it is wound into a roll.

1 Claim, 2 Drawing Figures METHOD FOR METALLIZING PLASTIC FILM BACKGROUND OF THE INVENTION This invention relates to the deposition of metal by the process of metal evaporation and subsequent condensation of the metal on a plastic substrate material, all in a vacuum. More particularly, it relates to the production of metallized dielectric plastics for use in capacitors, for which application the metals zinc and aluminum are most commonly used.

The process of zinc metallizing of rolls of plastic film and paper was developed in Germany more than 30 years ago. Typically, the vacuum chamber is provided with pumps capable of reaching a vacuum of about 30 microns within several minutes after starting pumpdown. The raw film or paper roll in the chamber is mounted on an unwind spindle and the film is then threaded through suitable tension-sensing and smoothing rolls, over a heated silver vapor source, then over a heated zinc vapor source, through a set of resistancemeasuring rolls and then onto a wind-up spindle.

The amount of silver vapor which condenses onto the film or paper is so small as to be difficult to detect or measure, but it is absolutely essential to the process since zinc vapor will not condense onto the film as a shiny, continuous layer unless the silver is there to nucleate the zinc deposit. The film being metallized may run at 200 to 800 feet per minute, depending on thickness of coating desired, chamber vacuum, silver vapor deposit, zinc source temperature and the nature or dryness of the film being coated.

The evaporation and deposition of aluminum requires a high vacuum, about two orders of magnitude higher than that required for zinc. Advances in the development of vacuum pumps have now made the large scale production of aluminum metallized film a common practice, where a vacuum of about 0.3 microns or less is normal in the metallizing zone of the vacuum chamber. Nucleating silver is not required and the coating may be run at a higher rate, e.g. 1500 feet per minute. Also, aluminum is more corrosion-resistant than zinc and its oxide is less conductive. As a result, aluminum is tending to replace zinc in the metallizing of film.

These two metals are now commonly used in metallizing many materials, such as paper, lacquered paper, polyethylene-terephthalate, polycarbonate, and polypropylene, for use in capacitors. For many years these film materials were metallized on only one surface. More recently a market has developed for materials metallized on both surfaces. Double metallizing is ac complished either by metallizing each surface sepa rately or by use of newer metallizers that are designed to metallize both surfaces simultaneously.

A roll of plastic film having been metallized on both surfaces often cannot be unrolled without the adjacent metallized surfaces sticking so that some metal from one surface is transferred to the adjacent surface. This problem is found to be most severe for the combination of polypropylene film double metallized with zinc. The transfer problem in the case of aluminum metallized material has not been experienced by us. It is theorized that some transfer does occur however, but for situations thus far encountered it has not been detected. For use in capacitors, double metallized film is usually required to have very thin and uniform metallized coatings, e.g. around 200 angstroms. This thickness represents only a few atomic layers of metal and any loss of metal by sticking and transfer can readily degrade the uniformity of the metal coating. The tendency for sticking and the transfer of metal varies greatly depending at least upon the material of the substrate film and the particular metal that is deposited on it.

It is therefore an object of this invention to greatly reduce or eliminate sticking and metal transfer that frequently occurs when a roll of double metallized film is unrolled.

It is a further object of this invention to produce rolls of double metallized film, whose metallized film quality and uniformity suffers no loss in the act of being unrolled.

SUMMARY OF THE INVENTION A roll of plastic film metallized on both film surfaces by a conventional vacuum metallizing process, often cannot be unrolled without the adjacent metallized surfaces sticking so that some metal from one surface is transferred to the other surface. According to the present invention, this sticking is greatly reduced or eliminated by modifying the conventional metallizing process so as to expose at least one freshly metallized surface to air prior to winding the double metallized film into a roll.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a schematic drawing representing a conventional continuous vacuum metallizer, modified to provide an air leak in the vicinity of a freshly metallized film surface according to a preferred embodiment of the present invention.

FIG. 2 shows a schematic drawing representing a conventional continuous vacuum metallizer, equipped for metallizing both surfaces of a substrate film simultaneously, and modified to provide an air leak in the vicinity of a freshly metallized film surface according to a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. I there is a simple schematic view of a modified conventional metallizer having only certain key elements shown. This metallizer is a type designed for metallizing only one surface of a substrate film at a time. Spindle 12 supports a supply roll 13 of plastic film which is to be metallized. The film l5 withdrawn from the supply roll I3 passes over a plurality of guide rollers l6, l7 and 18, then passes a few inches above a heated silver evaporator source 30, the silver source 30 containing a quantity of silver 31. The film 15 then passes a few inches above a heated metal evaporator source 32, the metal source 32 containing a quantity of metal 33. The film 15 subsequently passes over idler guide rollers or resistance measuring rollers 23 and 24, and around a laydown roller 26 which initially urges the film 15 into contact with a wind-up spindle 28 and subsequently into contact with the roll of plastic 29 that builds on the wind-up spindle 28. The wind-up spindle 28 is driven by a motor (not shown).

The silver source 30 and metal source 32, seen in cross section. are normally at least as long as the width of the film 15 being metallized. The sources are continuously heated so as to melt and continuously evaporate the silver 31 and the metal 33 such that the film l5 passing overhead becomes metallized by condensation on its bottom surface.

The above mentioned elements are all contained within a vacuum chamber 10. A vacuum pump (not shown) evacuates the enclosed space, zone 61 and zone 62, within the chamber 10. A more comprehensive description of the operating features of such a machine are given by McGraw in U.S. Patent No. 3,030,912.

In the case that the film 15 is to be zinc metallized, as was said, it has been found necessary to preevaporate minute amounts of silver. When the film 15 is to be coated with other metals, such as aluminum, the silver source 30 is not required.

In accordance with a preferred embodiment of the improved method of this invention, the sectional view of a tube 50 is shown in FIG. 1, which tube is at least as long as the width of the film l5, and which tube contains a row of holes only one 5] of which is shown, the holes being spaced at regular intervals along the tube 50. Air 53 is admitted to the tube 50, subsequently leaking through the holes, e.g. 51, and onto the freshly metallized surface of the film 15.

In this preferred embodiment the supply roll 13 of bare film is mounted on the spindle 12, the film being threaded among rollers as previously described and its end fastened to the wind-up spindle 28. The chamber is evacuated, the sources are heated, and the film is drawn from the supply roll, metallized on one surface and wound up in a roll 29. The vacuum is broken, roll 29 is removed and mounted again on spindle 12. The film is again threaded as before through the rollers and attached to spindle 28. The chamber is evacuated, the sources are heated, and the film is drawn from the roll on the spindle l2, metallized on the other surface, exposed to air being leaked from the tube 50, and wound up into a roll 29. The roll of double metallized film thus formed will unroll freely without the usual random transfer of metal from one surface to an adjacent surface.

In a second preferred embodiment, a film is simultaneously continuously metallized on both its surfaces and one of its freshly metallized surfaces is exposed to air just prior to the film being wound into a roll. In FIG. 2 is shown the organization of a machine having this capability. It will be noted that the machine of FIG. 2 differs essentially from that of FIG. 1 in that both surfaces of the film 15 pass over evaporation sources in the former.

In FIG. 2, spindle 12 supports a supply roll 13 of plastic film which is to be metallized. The film 15 withdrawn from the supply roll 13 passes over a plurality of guide rollers l6, l7 and 18, then passes a few inches above a first silver source 30 containing a quantity of silver 31 and over a first metal source 32 containing a quantity of metal 33. The film 15 then passes over idler guide or resistance-measuring rollers 19, 20, 21 and 22, and then passes a few inches above a second silver source 40 containing a quantity of silver 41 and over a second metal source 42 containing a quantity of metal 43. The film l5 subsequently passes over idler guide or resistance-measuring rollers 23 and 24, and around a laydown roller 26 which initially urges the film 15 into contact with a wind-up spindle 28 and subsequently into contact with the roll 29 that builds on the wind-up spindle 28. The wind-up spindle 28 is driven by a motor (not shown).

In accordance with this second preferred embodiment of the present invention, and with reference to FIG. 2, air 53 is leaked from within tube 50 through holes, e.g. 51, onto one of the freshly metallized surfaces of the film 15.

Again the silver sources 30 and are only required when the evaporating metals 33 and 43 are zinc. It will be understood that a machine of FIG. 2 compared to that of FIG. 1, will tend to expose the plastic film l5 to more heat. A normal technique for reducing the films exposure to radiated heat from the evaporation sources consists in the installation of heat shields indicated as 35 and 45 in FIG. 2. The heat absorbed by typical sheet metal heat shields may be conducted away to the body of the chamber and in some cases it may be desirable to equip them with adjacent heat exchanger tubes in which coolants such as water or liquid nitrogen flow. A typical alternate machine design for reducing the films exposure to heat as it passes directly over the sources, provides a large idler roller or drum directly over each source such that the film lies in contact with the drum in the region where metal is being deposited. Further, such drums may have coolants flowing through them.

These special methods for keeping the temperature of the film down become more important when low melting point films are employed, and when very low vacuums are required as in the case of aluminum metallization.

In both FIGS. 1 and 2 a baffie plate 25 is shown dividing the space within the chamber into two zones 61 and 62. A baffle plate is especially useful in the case of aluminum evaporation where a high vacuum of at least 0.3 microns is required in the evaporation zone 61. Air that has been trapped in the supply roll 13 continuously escapes as the roll unwinds. In the practice of this invention, it may become even more important since additional air is being released within zone 62 from the tube 50. The vacuum in zone 62 will typically read as high as 30 microns while in zone 61 it is 0.3 microns, suitable for aluminum metallizing. Slots are provided in the baffle plate through which the film passes.

In the second preferred embodiment it will be noted that only one metallized surface is exposed directly to the leaking air at a location along the route of the film lying between the region in the metallizer where metal is being deposited and the wind-up spindle. Both surfaces could have been so exposed to the leaking air but there are no apparent advantages in doing so.

It has been found that the desired results, as are achieved by the particular method of the two preferred embodiments described above, may also be achieved in a process wherein no modifications are required to the conventional metallizer. To wit, a film is metallized on one surface in a metallizer, the roll of film is removed from the metallizer and re-reeled in a normal air atmosphere so as to briefly expose the freshly metallized film (for about 3 seconds) over its entire length to air; the roll is reintroduced to the metallizer and subsequently metallized on the opposite surface. The final roll thus produced is free from sticking. This method has the disadvantages that it requires more time and is thus more expensive, and the extra handling tends to produce wrinkles in the film thus degrading the quality of the final roll of metallized film.

A roll of double zinc metallized polypropylene film was prepared in accordance with the process of the first preferred embodiment. The 10 inch wide 0.25 mil thick film was metallized on one surface under a vacuum of about 40 microns and running at a speed of about 315 feet per minute. A inch tube, slightly longer than the metallized film had been mounted horizontally in the metallizer, about one-eighth inch away from the freshly metallized film surface. The tube contained a row of 80 holes each 1/32 inches in diameter and spaced oneeighth inch apart. The holes faced the film.

After running by the tube, the film passed over an idler guide roller and was wound up on the horizontally mounted wind-up spindle. Air was bled into the metallizer through a valved pipe leading from one end of the tube to the air atmosphere outside the metallizer. The air bleed rate was 3 standard cubic feed per hour in one run and for another run reduced to 1.5 standard cubic feet per hour. In both runs the finished roll of double metallized polypropylene film was completely free of sticking when unwound, and the uniformity of the metallized surfaces greatly improved in comparison'to a similar roll having been produced without the benefit of the method of this invention.

We have concluded that the basic mechanism underlying the improvement realized by exposing metallized film to air before winding into a roll, is the oxidation of the metal surface so exposed. We believe that a metal oxide barrier is thus interposed between otherwise adjacent metallized surfaces of the film in the roll. The highly amorphous and relatively inert oxide barrier would reasonably have the effect of preventing adhesion between adjacent layers of film. Although this explanation is presented for purposes of clarity and completeness, applicants are not to be limited to their theory of the underlying roots and causes upon which the present invention relies.

What is claimed is:

l. A method for producing in a continuous vacuum metallizer a roll of double metallized thin polypropylene film for use in capacitors unrolling the film from a supply spindle, depositing silver and a subsequent coating of zinc to both sides of the film, and taking up the double metallized film on a take-up spindle, both spindles being located within the vacuum metallizer, wherein the improvement comprises exposing of only one of the metallized surfaces of the polypropylene to air at a location along the route of the film lying between the region in the metallizer where the metals are being deposited and the wind-up spindle. 

1. A METHOD FOR PRDUCING IN A CONTINUOUS VACUUM METALLIZER A ROLL OF DOUBLE METALLIZED THIN POLYPROPYLENE FILM FOR USE IN CAPACITORS UNROLLING THE FILM FROM A SUPPLY SPINDLE, DEPOSITING SILVER AND A SUBSEQUENT COATING OF ZINC TO BOTH SIDES OF THE FILM, AND TAKING UP THE DOUBLE METALLIZED FILM ON A TAKE-UP SPINDLE, BOTH SPINDLES BEING LOCATED WITHIN THE VACUUM METALLIZER, WHEREIN THE IMPROVEMENT COMPRISES EXPOSING OF ONLY ONE OF THE METALLIZED SURFACES OF THE POLYPROPYLENE TO AIR AT A LOCATION ALONG THE ROUTE OF THE FILM LYING BETWEEN THE REGION IN THE METALLIZER WHERE THE METALS ARE BEING DEPOSITED AND THE WIND-UP SPINDLE. 